code: def on_after_optimizers_step(self, trainer, optimizers):
      self.lrs.append(
          trainer.driver.optimizers[0].param_groups[0]["lr"]
      )
  
file_path: fastNLP/tests/core/callbacks/torch_callbacks/test_torch_warmup_callback.py
imports: import pytest
  import numpy
  from fastNLP.core.callbacks import TorchWarmupCallback, Callback
  from fastNLP import Trainer
  from tests.helpers.callbacks.prepare_trainer_args_for_torch_test import get_trainer_args
label_desc: Use image node_id 2 for calling the RecordLrCallback obj's underlying member method code with example usage: obj.on_after_optimizers_step(trainer, optimizers) without return types